This small PCB was designed to fit on top of the GPIO pins of your Raspberry Pi Zero, A+, B+, 2 and 3. It labels the pins clearly so that you don’t need to count pins. This is of great help for avoiding wiring errors and damage.
The board can either be placed it over the GPIO header or held right next to it. The latter approach will even work for the older A and B models of the Raspberry Pi, although there will be 14 additional labels which can safely be ignored.
The labels on one side of the board denote the GPIO pin numbers 1 through 40, specifying Ground (GND) and Power (5V, 3V3), whereas on the other side the labels correspond to the GPIO numbers on the SoC, so it can be as useful as possible!